Wireless communication becomes the most popular communication means in the communication society. Integrated circuits and devices supporting radio frequency communication need to be consistent with various standards in different frequency bands. In order to allow multi usage of components for different frequencies, as well as for different standards, the components need to be able to vary their tuning frequency.
One example for radio frequency communication is radio-frequency identification (RFID) technology. RFID is an upcoming technology in many applications. RFID readers are used to locate and readout RFID tags with a unique identification code. The RFID tags can be used as smart labels in stores on products for identification, localization, stock checking and pricing of products. Tracking products or people, in particular in logistic technology, will use RFID technology. Further, RFID tags may be implanted into the human body for identification as well as for medical reasons. Furthermore, access to restricted areas or vehicles may be controlled by RFID technology.
The advantage of the usage of RFID technology, for example compared to conventional barcodes, are dynamic product information, no line-of-sight being required, no human intervention for scanning being required, multiple tag read-outs are possible at the same time, re-usability, durability, and robustness. Obiquitous use of RFID products is expected when technology will be cheap.
The cost of RFID technology is thus a major concern in this field. Cost increasing factors are considered. Different standards exist for RFID readers, RFID writers, as well as for RFID tags. For example, globally multiple operating frequencies are used in current RFID technology. The currently used operating frequencies may be subdivided in four different groups: 1) a low frequency group, which has a frequency range of 30-300 kHz, 2) a high frequency group, which has a frequency range of 3-30 MHz, 3) an ultra high frequency group, which has a frequency range of 300 MHz-3 GHz, and 4) a radio frequency group, which has a frequency range of 2-30 GHz. These four groups are again split in frequency bands for short-range devices, which bands are controlled in the European Union, the US, and other areas of the world. Further, changes to the frequency ranges may apply in the future due to political or economical reasons. For example, frequency ranges may differ depending on country or use case. A higher frequency may result in an increase of the data transmission speed, and extends the signal path from the RFID reader to one or more RFID tags. For example, low frequency and high frequency bands may cover signal distances of 1.5 m at data rates of 25 kbit/s. The ultra high frequency as well as the radio frequency bands may allow signaling up to 10 m at data rates of up to 100 kbit/s.
In view of the above, it is desirable to have re-configurable RFID readers/writers and/or tags, which allow covering a multitude of frequency bands. One constrain, however, is that the RFID tag as well as the RFID reader needs to be cheap in order to allow the RFID technology to become a mass market product. Moreover, in particular relevant for the RFID tag, the drive circuit for receiving and sending within the frequency bands needs to be small in size.
In order to allow for reconfiguration of the RFID reader/writer or the RFID tag, a transmission-reception front-end may be provided, which is re-configurable in view of the transmission and reception frequency. Most of today's tunable filters however, rely on either mechanical or electronic tuning using varactor diodes or switched capacitors. Mechanically tunable cavity filters are not suitable for RFID tags due to their size. Integrated varactor-diode based tunable filters are faster, however, they provide high losses at radio frequency due to their low quality factor. Switched capacitor filter banks are usually big in size.